The invention pertains to a base material for the production of blank blades, particularly for circular saws, cutoff wheels, gang saws, as well as for cutting and scraping devices, consisting of a base steel enriched with carbon starting from its surface consisting of two broad surfaces, two end face surfaces and two long edge surfaces, wherein the base steel has a basic carbon content of less than 0.3 wt % carbon.
It is conventional to use tool steels with a carbon content between 0.5-1.0 wt % or low-alloyed structural steel (as steel for tempering) in order to produce a base material for the production of blank blades, particularly for circular saws, cutoff wheels, gang saws, as well as for cutting and scraping devices. The heat treatment of these materials is then done with the objective of obtaining a homogeneous texture and a uniformly high hardness over the entire thickness range. The necessary toughness of the base materials is achieved by a controlled tempering, the latter, however, necessarily being connected to losses of hardness. Depending on the purpose of use and the specific load on the base material, for saws, for instance, hardness values between-roughly 37-50 HRC are produced.
Particularly in the hot-rolling process of a typically used tool or tempering steel and in the austenitization treatment of it for hardening, the carbon diffuses out of the boundary layer of the material. A decarbonization of the surface results, so that the decarbonized boundary layer with low hardness has to be ground away after heat treatment.
In order to improve service life, a large number of saws are hard-chrome-plated, tipped with hard metal or diamonds or stellitized. The tipping is done by soldering or sintering. These measures lead to clear improvements of service life without, however, influencing the inherent strength of the blank blades. The manufacturing costs of these saws are markedly increased by the measures for increasing service life. This necessarily leads to a reduction of the teeth or number of segments, which worsens the cutting quality and increases the noise emission.
In the corporate publication xe2x80x9cSie+Wirxe2x80x9d of the Stahlwerke Sxc3xcdwestfalen, No. 14/1975, manufacturing processes for various types of saws are described, reference being made to the fact that there is always a demand for a sheet which is as free of strains as possible with low decarbonization values and homogeneous texture formation. The steels used must have a very fine-grained texture with good tenacity after hardening and tempering, so that the very high centrifugal and shearing forces that appear can be securely absorbed.
The typification of the saws in the aforementioned corporate publication relies on a customary division into three groups, corresponding to the material to be cut. According to the material group, different requirements are placed on the properties of the saws. These groups are:
1. saws for wood and plastic (circular wood saws, hard metal tipped circular saws, forestry and gang saws;
2. saws for metal (segmented circular saws, cutoff saws, circular hot sawing machines);
3. saws for stone (diamond-tipped circular saws, diamond-tipped slab saws).
One of the requirements of saw blades is the presence of a high bending stiffness or shape stability. To stabilize slab, band, circular, and quick-cutting saw blades as well as diamond discs, in particular to compensate for strains produced by nonuniform heating in the tool body, a known procedure consists in producing internal strains in certain zones deliberately by tensioning the blade (xe2x80x9cComparative studies on the tensioning of circular saw blades with machines and flattening hammers,xe2x80x9d in the special issue of Holz als Roh- und Werkstoff, Vol. 21 (1963), pp. 135-144). Such a generation of internal strains can be accomplished in hardened steel disks or bands by cold hammering with a hammer or mechanically by rolling or pressing, but in any case, it represents an elaborate processing step in manufacturing.
The thermochemical enrichment of iron and steel materials with carbon has been known for some time, and is referred to as case-hardening; If nitrogen is introduced into the material at the same time, one speaks of carbonitriding. An overview of caburizing, with special emphasis in regard to a mathematical modeling of it, is provided, for instance, by the article xe2x80x9cThe carburizing processxe2x80x9d in Hxc3xa4rterei Technische Mitteilungen, Vol. 50 (1995) No. 2, pp. 86-92. The carburizing process can take place in a gaseous medium, in a salt bath or in powder and is generally performed at temperatures between 900-1000xc2x0 C. As carbon donors, agents are employed here whose carbon activity must be higher than that of the iron material. The carbon emitted from the carburizing agent diffuses into the boundary layer of the workpiece to be carburized. A characteristic carbon concentration profile results, according to the selected process parameters, such as temperature and treatment time, as well as the carbon activity of the carburizing agent and the composition of the iron material. The carbon concentration declines continuously with increasing distance from the boundary, until it reaches the initial level of the material in the inside of the material. The carburizing depth At is to be considered a characteristic parameter of significance for practice in this regard. The carburizing depth At is defined as the vertical distance from the surface up to a boundary characterizing the thickness of the layer enriched with carbon. The carbon content at which this boundary is assumed to exist is subject to standardization (cf. DIN EN 10 052) and is generally agreed to be 0.35 wt % carbon. The carburizing depth At of a material increases with increasing duration of carburizing of a workpiece, the geometry of the latter also playing a role. For convex-curved workpiece surfaces, at edges or points, therefore, a greater carburizing depth At occurs, since a comparatively smaller volume-is available to the carbon diffusing in from all sides. Thereby an excess carbonization can occur, which is characterized by the separation of carbides or by an undesired residual austenite content after hardening.
A method of this class for producing highly alloyed strip steel which is used for quick-cutting and tool steel as used for, among other things, the purpose of manufacturing blades and cutters found in razor blades or metal saw blades, has become known from DE-OS 2,431,797. The high content of alloy elements and the type of alloy elements, e.g. 12-13 wt % chromium, whereby a high hot hardness can be achieved, corresponds to this purpose of the strip steel for metal saws or razor blades, classified in the second group-according to the division above. Highly alloyed steels with additional high carbon content are difficult to process using hot and cold rolling in the manufacturing process, i.e., they are at risk for cracking and fracturing. Therefore a strip material with low carbon content is first either sintered or cold-rolled and subsequently enriched with carbon, either over its entire surface or partially, in the edge area. The carbon enrichment is done over the entire cross section or thickness of the strip material. Thus a carbon concentration corresponding in its level to the carbon concentration of tool steels results with almost a constant profile over the entire thickness of the strip material, slight corresponding to the foreseen usage of the material.
From AT-PS 372,709, a cutting tool, specifically a saw, made of alloyed steel is known, which is enriched in the area of its working surfaces or teeth with 1.8-2.2 wt % carbon to a depth of 0.02-0.10 mm, the carbon content at a depth of 0.15-0.25 mm reaching the carbon content. The steel alloy consists of iron with the unavoidable impurities and contains 0.1-0.3 wt % carbon, 0.2-2.0 wt % silicon, 0.5-1.5 wt % manganese, 5.0-7.0 wt % chromium, 1.0-2.0 wt % tungsten, 1.0-2.0 wt % molybdenum, 0-2.0 wt % vanadium, 0-0.5 wt % titanium, and 0-0.5 wt % niobium. To produce the cutting tool, the workpiece blank, specifically, the saw blade, is subjected to a case-hardening at temperatures in the range of 850-1050xc2x0 C., whereafter the hardening in air, oil or in a hot bath takes place. The slight carburization depth At and the strong case hardening lead to a carbon gradient from the broad surface to the area not enriched with carbon of roughly 6-14 wt % C/mm in the boundary area of the base steel. In this way it is intended, in particular, for a surface layer of elevated wear resistance to be achieved. The alloy employed is a special steel, corresponding by its content-of alloy elements to a high-speed steel, but without having a correspondingly high carbon content. The carbon content here is typical of carburized steels , but the alloy content is atypical. The use of such a material pursues the goal of replacing fast-machining-steel by the alloy specified and treated in the manner described. Here too, similar to the method corresponding to DE-OS 2,431,797, a reduction of the manufacturing costs by reducing the risk of rejects and a savings in material by avoiding an overuse of strip steel in its forming processes is intended. In the process, a high hot hardness in the workpiece can be achieved, which is characterized by tempering temperatures of 500xc2x0 C. and more. Given the core hardness of the material, a value of 45-55 HRC can be assumed, as with fast-machining steels.
A disadvantage of this cutting tool and its manufacturing method consists in the fact that band saws are expressly out of the question, presumably, because the necessary tensile and reversed bending fatigue strength cannot be achieved. As workpiece blanks, moreover, keyhole saw blades are produced by stamping, milling and setting the teeth, and are only thereafter case-hardened, hardened and tempered. It must be assumed however, that after this treatment the saw blades can no longer have their teeth set, because of their high carbon content. Because of the case hardening taking place omnidirectionally, moreover, an excessive carbon enrichment may occur in certain edge areas, as described above, which has an unfavorable effect on the edge properties and strength of the teeth because it causes embrittlement of the material.
The invention is based on the problem of specifying a base material of the generic type with which blank blades for circular saws, cutoff disks, gang saws and cutting and scraping devices with enhanced component strength can be produced while avoiding the formation of a decarbonized edge area, wherein a higher hardness at the surface is possible to increase wear resistance with identical operating and fracture safety and the noise emission in the operating state is reduced. It should furthermore be possible for untipped saws for wood and plastic, such as circular wood saws, forestry or gang saws to be produced from this base material, which are distinguished by a long service life with low production expense.
This problem is solved by a base material of the generic type in which the base steel has boundary areas enriched by a thermochemical treatment with 0.5-1.1 wt % carbon starting from a least one outside surface and making a transition with decreasing carbon content to an area not enriched at all with carbon, or only enriched slightly, while, at the edge surfaces, the base material has the sandwich structure formed by the boundary area enriched with carbon and the area not enriched with carbon. The thermochemical treatment is preferably a carburizing process, but can also advantageously be a carbonitriding process, if the carburizing medium contains nitrogen or nitrogen compounds, such as ammonia. The nitrides formed in this manner in the base material according to the invention cause an additional elevation of wear resistance and counter fatiguing of the material.
In this manner, the tool steels with a high degree of purity that are ordinarily used can be can be replaced by the base material according to the invention, whose base steel, preferably low-alloyed or unalloyed structural steel, need not meet these purity requirements. Special steels are not required as starting materials, which implies a reduction of the steel manufacturing costs. With the base material according to the invention, it is not only for an elevated wear resistance to be achieved at the broad surfaces, but also a greater component strength, characterized by a greater bending strength, static bending strength and reversed bending strength.
The base material can advantageously also have a sandwich structure which consists of a broad surface enriched with carbon, an inner core not enriched or only slightly enriched with carbon and an additional broad surface of the base steel enriched with carbon. After the production of saws, cutoff disks or cutting devices, this structure is then also present on the saw teeth or blades. With repeated use of the tool, an uneven wear results over the thickness of the material, specifically a so-called cratering. That is to say, the hard and wear-resistant broad surfaces wear more slowly than the core which is not enriched with carbon, whereby the edge surface obtains a concave shape and a self-sharpening effect occurs in the cutting area.
It has been shown that, since the physical properties of the base material can be gradually changed by differing carbon contents, it is of particular advantage for the wear and strength properties to be achieved in the blank blades if the quotient of a carburizing depth At of the edge area, in which the carbon content is 0.35 wt %, and the thickness of the base material had a value of 0.15-0.40. The depth of the carburized area can preferably be chosen such that, after hardening and tempering of the thermochemically treated base material, at most roughly ⅓ of the total depth of the base steel has essentially the original hardness of the base steel or a slightly higher hardness, and at least roughly ⅔ of the thickness of the base material has a higher hardness. In particular, it is preferred that, after hardening and tempering of the thermochemically treated base steel, at least roughly 50% of the thickness of the base material has essentially the original hardness of the base steel or a slightly higher hardness, and at least roughly 50% of the thickness of the base material has a higher hardness. After hardening and tempering, the hardness of the broad surfaces of the base material advantageously lies in the range of roughly 50-63 HRC preferably in the range of 55-60 HRC and, in the area not enriched with carbon, 20-40, preferably, 30-35 HRC. The enrichment of the base steel with carbon on both sides over the entire broad surface of the steel sheet, but the carbon enrichment can also be conducted only partially, in the later toothed area of the on both sides, or partial areas, at subsequent soldering areas or the like, can be provided which are excluded from carbon enrichment. The areas not enriched with carbon or only slightly enriched consist after hardening and tempering of a-mixed structure of ferrite and perlite and/or of bainite, preferably in its lower stage.
Thus it is possible, with low requirements on the base steel for saws to be produced which consists of a steel sheet that is enriched with carbon on both sides, for instance, or only partially, by means of a thermochemical treatment, particularly carburizing. Surprisingly, it was established that, with a base steel having a very low carbon content of 0.1-0.2 wt % and subsequent case hardening as well as hardening and tempering, saws can be produced with better quality which have no linear/hardness strength profile with respect to their thickness and surface. The boundary area enriched with carbon here favorably has a mean carbon gradient of roughly 0.25-0.75 wt % C/mm, preferably 0.40-0.50 wt % C/mm for the surface to the area not enriched with carbon.
While conventional saws have a martensitic structure throughout with homogeneous properties, this is present in the saws produced from the base material according to the invention only at the surfaces of the areas enriched with carbon. The requirements for toughness are largely met by the softer core, while, in case of a saw that is not tipped or stellitized, the surface determines the good cutting properties and high stability of the saw with its hardness.
As already presented, low- or nonalloyed structural steels are preferred as base steels for the base material according to the invention. Thus all steels that can be used, alloyed or unalloyed, as hardened steels are suited for the base material according to the invention. Heat-treatable steels with low carbon contents, as well as rust- and acid-resistant steels with an elevated chromium content (12-13 wt %) can likewise be used. In Table I, such steels that can be utilized according to the invention are presented by way of example, without the invention being limited to these, however.